1. Field of the Invention
The present invention relates to semiconductor wafer processing, and specifically to the detection of conductive solutions in semiconductor fabrication equipment used to process semiconductor wafers. More specifically, the present invention relates to detecting conductive fluids, including copper plating solution, which may destructively interact with the wafer during fabrication.
2. Description of Related Art
The semiconductor wafer fabrication process is vulnerable to destructive processing fluids that may inadvertently come in contact with the silicon wafer. Metal-containing fluids, a by-product of the fabrication process, have been shown to be extremely harmful to the wafer. In the instance of copper plating, the unintentional transfer of copper via plating solution to the back of a wafer can cause contamination in other process equipment, exposing a semiconductor wafer to contamination and degradation. This contamination can potentially shutdown an entire production line while steps are taken to eliminate the contaminant and mitigate the damage caused. Migration and diffusion of copper into the silicon wafer destroys the silicon devices during fabrication. Current fabrication tools, such as the SABRE Copper Electrofill System, and the like, do not currently make use of an apparatus or method for sensing acidic fluid excursions, such as copper sulfate.
Most semiconductor fabrication processes are automated with robotics to ensure safe, reliable, and efficient production techniques. The automated machinery includes a robotic arm to hold, manipulate and control the wafer, i.e., an end effector. The end effector is the chief handling feature of the robotic instrument. Wafers can be subjected to contaminants during handling and processing. Enhancing production efficiency while eliminating unwanted contaminants during wafer handling presents technical challenges to equipment modifications and implementation.
During normal operation, it is desirable to have copper plating solution contained within and by the plating cells. However, backside contamination of the wafer by copper sulfate may happen through a number of means. In the Sabre Copper Electrofill System, for example, a semiconductor wafer is taken out of its cassette, and placed face-side down in the clamshell. The clamshell is a fixture used to hold the wafer during copper electroplating. The clamshell clamps the wafer, seals the periphery of the face, makes electrical contact with the wafer, and seals the wafer's backside. When the wafer is dropped into the plating solution, it becomes vulnerable to contamination. One form of contamination, called Lip Seal or Clamshell contamination, occurs when the seal around the outer diameter of the wafer, i.e., the lip seal, fails. The clamshell submerges into the plating cell during the electroplating process. Wafers are transferred to and from the clamshell and secured to the clamshell using a vacuum. The solution commonly used for electroplating copper, acid-copper, is composed primarily of dilute sulfuric acid (˜0.01–20%), copper ions in the form of copper sulfate (15–55 g/l), chloride ions in the form of HCl (10–100 ppm), and ppm-levels of organic additives to control film properties. The plating solution may traverse around the seal in a failure mode, and ultimately come in contact with the edge of the silicon wafer. The copper in the plating solution can contaminate the wafer upon contact. In this failure mode, most of the copper sulfate will be concentrated on and about the circumference of the wafer. The robot end effector could then be contaminated when picking the wafer from the clamshell after the failure. This type of failure is a potentially destructive failure mode that must be addressed during the fabrication process.
A second form of contamination is spray out from plating cells. A plating cell is a reservoir that contains copper sulfate. The clamshell submerges into the plating cell during the electroplating process. Spray out is a main pathway for contaminating the end effector on a regular basis. The wafer is rotated after plating at high speed. During the wafer rinse, the rotations can generate a mist, or spray out. This spray out can cause copper contamination on the end effector at any location in the system. Normally, the spray out is contained at the plating cell shields; however, when this containment breaks down, destructive contamination can occur. One safety mode that may be employed requires software algorithms to command the fabrication robot to retreat to a safe location between processes. In this situation, the end effector is flipped when the robot begins an idle mode.
The robotic instrument itself can become contaminated once copper sulfate is introduced into the vacuum chamber. The robotic arm or end effector uses an injection of nitrogen (an “N2 puff”) to release the wafer; however, the nitrogen gas is capable of carrying copper particles and placing them on the backside of the wafer. Usually, when the end effector reaches this state of contamination, excessive spray out or lip seal contamination has already been realized.
Edge or backside copper contamination is an unwanted by-product of the semiconductor wafer fabrication process. A sensor that detects the presence of copper plating solution and other conductive solutions can reduce the likelihood of this kind of contamination.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a sensor that can be easily adapted to an end effector of an automated semiconductor process robot for detection of conductive fluids.
It is another object of the present invention to provide a sensor that is capable of covering the acid path for both sides of an end effector of a semiconductor process robotic instrument.
It is a further object of the present invention to provide detection of copper sulfate at the source of the contamination in a semiconductor wafer fabrication process.
It is yet another object of the present invention to provide a sensor for a semiconductor wafer processing system capable of detecting failure from clamshell or excessive spray out from the plating cell.
It is a further object of the invention to provide for a method of operation to eliminate acid contamination in a wafer transfer robotic arm.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.